In terms of a power device, between an on resistance and a reverse blocking voltage, there is a trade off relation which is specified, in principle, by a forbidden band gap. Therefore, in the current Si power device, obtaining high performance beyond the theoretical limit determined by the forbidden band of Si is of difficulty. However, making the power device by a semiconductor material having a wide forbidden band gap can greatly relieve the conventional trade off relation, thereby realizing a device which is remarkably improved in at least one of the on resistance and the reverse blocking voltage.
With the temperature so increased as to boomingly generate a electron-positive hole pair by thermal excitation, a semiconductor is unable to distinguish p type area from n type area or to control carrier density, making it difficult to operate the device. In the case of an Si semiconductor having a forbidden band gap of 1.12 eV, generation of the electron-positive hole pairwise is intensified from around 500 K (=227° C.), therefore, a practical upper limit temperature as a semiconductor device is 180° C. on the premise of a continuous operation. Making a semiconductor device (not limited to a power device) using a wide forbidden band material will greatly increase an operating temperature area (for example, more than or equal to 300° C.), greatly widening application of the semiconductor device.
The silicon carbide (hereinafter denoted “SiC”) semiconductor under the present invention is one of the wide forbidden band semiconductor materials capable of improving performance. Recently, with the development of single crystal substrate, a wafer (3C, 6H, 4H) featuring a comparatively good quality and having a diameter of more than or equal to three inches is commercially available. SiC has a forbidden band gap, specifically, 3C crystal system having 2.23 eV, 6H crystal system having 2.93 eV, and 4H crystal system having 3.26 eV, each sufficiently wider than those of Si. Compared with other wide forbidden band semiconductors, SiC is chemically stable extremely and mechanically rigid. With the SiC semiconductor, forming of pn junction, controlling of impurity density and selectively forming of impurity area are possible in a method like that for producing Si semiconductor.
In addition, SiC is especially outstanding over other wide forbidden band semiconductors. Specifically, like Si, SiC is a unique semiconductor capable of generating oxide silicon (SiO2) by thermal oxidizing, which is an advantage. With this, it is expected that a normally-off type MOS drive device, for example, a power MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) or power IGBT (Insulated Gate Bipolar Transistor) can be realized by the SiC, making various companies vigorously develop the SiC.
Realizing the MOS drive SiC device, however, may cause various problems. Among other things, a drastic improvement in reliability of gate oxide film is the greatest issue. Primarily, an SiC thermally-oxidized film has the following features: (1) an energy barrier against a conductive electron of SiO2/SiC interface is, in principle, smaller than that of an Si thermally-oxidized film, and (2) a considerable amount of C (carbon) as a remnant is included in SiO2. Thus, it was expected that the SiC thermally-oxidized film, in principle, causes more leak current than the Si thermally-oxidized film and finds difficulty in bringing about as high reliability (root cause) as that brought about by the Si thermally-oxidized film. However, reliability of an actual SiC thermally-oxidized film is below the above expectation, causing further deterioration.
Reasons therefor are to be explained. The Si device is known for that an Si thermally-oxidized film formed by thermally oxidizing a substrate having a surface with crystal imperfection (dislocation and the like) causes an insulation breakdown in a low electric field or is remarkably decreased in time dependent dielectric breakdown (TDDB) lifetime. The SiC thermally-oxidized film may cause the like effect. In “Tanimoto et al., Extended Abstracts (The 51st Spring Meeting, Tokyo University of Technology, 2004); The Japan Society of Applied Physics and Related Societies, p. 434, Lecture No. 29p-ZM-5 (hereinafter referred to as “non-patent document 1”)”, the present inventors (Satoshi Tanimoto is the present inventor) reported the following: TDDB lifetime of a gate oxide film of a power MOSFET having a practical area depends on defect attributable to a great amount of dislocations on the surface of the SiC substrate used. As a result, compared with the Si thermally-oxidized film (having no same defect), the SiC thermally-oxidized film has TDDB lifetime decreased by more than or equal to two-digit.
Use of a layered (gate) insulating film may solve the above reliability problems of the SiC thermally-oxidized film, although not so much reports have been made thereon. Among the above, an ONO gate insulating film is the most desirable and practical. In “ONO”, “O” denotes SiO2 film (oxide silicon film) and “N” denotes Si3N4 film (silicon nitride film. Otherwise, denoted “SiN film” for short).
In “IEEE Transactions on Electron Devices, Vol. 46, (1999). p. 525” (hereinafter referred to as “non-patent document 2”), L. A. Lipkin et al. studied reliability of a metal-insulator-semiconductor (MIS) structure of a gate electrode having the following structure:
Between i) an n+ type 4H-SiC substrate (having a surface where an n− type epitaxial layer is grown) and ii) a Mo/Au gate electrode, an ONO gate insulating film is sandwiched which includes:                1) SiC thermally-oxidized film,        2) an SiN film produced by an LPCVD (Low Pressure Chemical Vapor Deposition), and        3) an SiO2 film formed by thermally oxidizing the surface of the above SiN film in 2).        
The study by L. A. Lipkin et al. has obtained a maximum insulation breakdown strength BEox=about 13.1 MV/cm (SiO2 converted), and a maximum stress current strength BJox=about 0.25 mA/cm2.
Herein, the superscript “+” and the superscript “−” on “n” or “p” each denoting conductivity (negative or positive) of the semiconductor denote, respectively, high density and low density.
Meanwhile, X. W. Wang et al., in “IEEE Transactions on Electron Devices, Vol. 47, (2000) p. 458” (hereinafter referred to as “non-patent document 3”) discloses an evaluation on reliability of an MIS structure where an ONO gate insulating film formed by thermal oxidizing of a surface of SiO2/SiN films layered by a JVD (jet vapor deposition) is sandwiched between a 6H-SiC substrate and an Al gate electrode, to thereby obtain BEox=about 12.5 MV/cm (SiO2 converted) and BJox=3 mA/cm2.
However, the above two ONO gate insulating films according to the non-patent document 2 and the non-patent document 3 each are less reliable than the SiC thermally-oxidized film. Actually, in “Material Science Forum, Vols. 433-436, (2003) p. 725” (hereinafter referred to as “non-patent document 4”), the present inventors Satoshi Tanimoto et al. report an accomplishment of BEox=13.2 Mv/cm and BJox>100 mA/cm2 by using a MOS structure including a thermally-oxidized film of 4H—SiC substrate. As obvious by comparison with the result of the non-patent document 4, the above two ONO gate insulating films according to the non-patent document 2 and the non-patent document 3 are less reliable than the SiC thermally-oxidized film obtained by the present inventors in terms of BEox and BJox.
Under the above background, recognizing potentiality of the ONO gate insulating film, the present inventors studied a method for applying the ONO gate insulating film to a structure or production processes of an actual power MOS device. By the following operations, the present inventors successfully accomplished BEox=21 MV/cm and BJox>10A/cm2 which are far better in performance than those of the above two ONO gate insulating films according to the non-patent documents 2 and 3 and the conventional SiC thermally-oxidized film in the non-patent document 1:
1) Between a polycrystalline silicon gate electrode and an SiC substrate, sandwiching the ONO insulating film where i) an SiC thermally-oxidized film ii) CVD silicon nitride film and iii) a thermally-oxidized film of the CVD silicon nitride film in ii) are sequentially layered.
2) Providing the polycrystalline silicon thermally-oxidized film and the silicon nitride sideface thermally-oxidized film, respectively, on a sideface of the gate electrode and a sideface of the silicon nitride film.
Refer to “Satoshi Tanimoto et al., Material Science Forum, Vols. 483-485, (2005) p. 677”, hereinafter referred to as “non-patent document 5”. This ONO insulating film structure has TDDB lifetime (=charge quantity passing per unit area until insulation breakdown) of QBD=about 30 C/cm2 which is at least two-digit higher than that of the SiC thermally-oxidized film, and is substantially equivalent to that of a thermally-oxidized film on a non-defect single crystal Si substrate.